The work of specifying the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) consisting of the Long Term Evolution (LTE) and System Architecture Evolution (SAE) concepts is currently ongoing within the 3rd Generation Partnership Project (3GPP).
One important focus area in E-UTRAN standardization work is to ensure that the new network is simple to deploy and cost efficient to operate. The vision is that the new system shall be self-optimizing and self-configuring in as many aspects as possible. One aspect that benefits from self-optimization and self-configuration is the management of the random access channel (RACH).
During initial access, the UE seeks access to the network in order to register and commence services. The random access (RA) serves as an uplink control procedure to enable the UE to access the network. Since the initial access attempt cannot be scheduled by the network, the RA procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. Including user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore, it has been decided to separate the transmission of the random access burst (preamble), whose purpose is to obtain uplink synchronization, from the transmission of user data.
Preambles in LTE are based on Zadoff-Chu sequences. A Zadoff-Chu sequence of length N can be expressed, in the frequency domain, as
                                          X            ZC                          (              u              )                                ⁡                      (            k            )                          =                  ⅇ                                    -              jπ                        ⁢                                                  ⁢            u            ⁢                                          k                ·                                  (                                      k                    +                    1                                    )                                            N                                                          (        1        )            where u is the index of the Zadoff-Chu sequence within the set of Zadoff-Chu sequences of length N=838. Out of one Zadoff-Chu sequence—in the following also denoted root sequence—multiple preamble sequences may be derived by cyclic shifting, were a shift is given by su,v(n)=su(n−vNCS mod N), where su is the inverse discrete Fourier transform (IDFT) of XZC(u)(k).
Root sequence planning may be done manually, using tools or other means when deploying or re-planning the network. This approach is, however, not satisfactory due to several reasons.
Firstly, there is a need to perform extensive field trials, which is very costly. Secondly, root sequences must be re-planned if network characteristics changes, e.g., coverage changes or the cell needs to be set to high-speed mode. And, thirdly, using tools or field trials is a slow process and not sufficiently responsive to changes in network, hence, it may take a while before a good allocation is performed.